Electrostatic printers and copiers commonly use an electrostatic writing head to form a latent image of electrical charges onto recording media having a dielectric and conductive layer, commonly referred to as a paper web. The paper web carrying the latent image is then directed to a liquid toning applicator which deposits oppositely charged toner particles onto the paper web, thereby developing the latent image.
An important consideration in controlling image quality is positioning of the charge deposited on the paper web. This is necessary considering that multiple colors are applied to a given area of the paper-web by successive applications of a latent image and toner particles. Wide-format electrostatic printers use a full-width print head to deposit a latent charge on dielectric media. To obtain high resolution printing, e.g., 400 dpi, a large number of electrodes must be employed so that the print head is coextensive with the width of the paper web. For example, a 36 inch wide print head would require 14,400 electrodes. A drawback with the full width print head is that its width is fixed. This may prove problematic in areas that have varying humidity, because the dimensions of the paper web change proportionally to the level of humidity present.
One manner in which to overcome the drawback with full width print heads is to operate the electrostatic printer in a humidity controlled environment. Alternatively, the paper web may be placed in an existing environment for a time sufficient to allow the paper web to stabilize. In this fashion, no further dimensional changes in the paper web would occur due to environmental humidity. Both of these approaches increase the cost to produce electrostatic prints.
Many prior art devices have been employed to compensate for dimensional changes in a substrate due to variances in ambient humidity. U.S. Pat. No. 4,949,104 to Negoro et al. discloses a justification system for a printer employing a continuous recording form. The justification system includes, inter alia, a timing pulse signal generating system which creates timing pulses in synchronism with traveling sprocket holes formed into the continuous recording form. The timing pulses signal the commencing of the printing on each printing segment, as well as the termination of the advancement of the continuous recording form. This system allows positional placement of the continuous form along a direction parallel to the direction of the form's travel so that printing may be accurately placed thereon.
U.S. Pat. No. 4,772,901 to Boyer et al. discloses an electrostatic printer employing dehumidified air. The printer includes, inter alia, an ion modulated electrostatic print head for forming latent electrostatic images, a dielectric imaging member comprising a layer of dielectric material, means for developing the latent electrostatic images on the dielectric imaging member, means for supplying unheated dehumidified air having a relative humidity of less than about 20% and directing the same onto the dielectric imaging member.
A problem encountered with the aforementioned inventions is that neither electrostatic print apparatus allows compensation for dimensional changes in the widthwise direction of the dielectric material. What is needed is a scanning electrostatic printer that compensates for dimensional variations in the dielectric material by allowing spatial adjustments in the placement, thereon, of an electrostatic image along two dimensions.